Viral products which interact with host components in order to suppress the host's defense mechanisms are major virulence factors. The elucidation of these virus-host interactions is necessary in order to understand the molecular mechanisms responsible for viral pathogenicity. We have recently shown that the non-structural protein of influenza A virus (NS1) plays a role in overcoming the type I interferon-mediated antiviral response of the host by preventing type I interferon synthesis. In this proposal, the molecular mechanisms by which the NS1 protein inhibits the interferon system during viral infection will be studied by comparing the phenotyping characteristics of wild-type influenza A and B viruses with those of recombinant viruses containing altered NS1 genes. The inhibitory effects of the NS1 protein on two transactivators which are known to be involved in type I IFN synthesis (IRF-3 and NfkappaB) will be characterized. The functional contributions of the different domains of the NS1 protein on (1) prevention of interferon synthesis, and (2) virulence in mice will be studied using recombinant influenza viruses containing different mutations in their NS1 genes. These experiments will expand our knowledge of the biological functions of the NS1 protein during the replication cycle of influenza A and B viruses, particularly with respect to its interaction with the interferon system, and they may offer approaches for the design of antivirals against influenza viruses targeting the NS1-host interactions. In addition, recombinant influenza viruses bearing mutations/truncations in their NS1 gene will be characterized in mice. Stable mutations resulting in an optimal balance between attenuation and immunogenicity will be identified. The results will tell us whether recombinant influenza viruses with defects in their interferon antagonist genes (NS1) represent potential live attenuated vaccines against influenza.